WIN32 support for path concatenation
WIN32 support for path concatenation

Summary:
So far, Flipper has concatenated paths assuming is only ever built on unix-like systems.

For RNW, this approach fails.

This change is a very simple way of achieving WIN32 support without using any extra libraries or routines for path building.

Reviewed By: antonk52

Differential Revision: D39053479

fbshipit-source-id: e8b4b71cc1d17ca6aba88d40cad7d96a30ad1267

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